Let's hope we NEVER need to use this helpful trick, but I'm so glad I came across it just in case. Thanks to Life Aspire:
It's time to dive in and learn this helpful hack. The first thing you'll do is wait until the person who zip-tied you is out of earshot. Then, untie your shoelaces, loop them through the zip-tie and tie the laces together. Lastly, take your feet and begin to rub the lace back and forth against the tie. The friction caused by rubbing will hopefully burn through the tie and your hands will soon be free!
Watch the video to see a demonstration: